We are studying the biological effects of natural antioxidants present in soybean and commercial soybean processing by-products. These agents can repress the formation of free radicals and also express antimutagenic activity. We examined the antioxidant and antimutagenic activity of a fraction (PCC) of a commercial soybean processing by-product. The effect of PCC on free radicals was examined by electron spin resonance spectroscopy using the spin trapping agent 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). 5.5 [unreadable]g/[unreadable]l PCC inhibited 50% of OH[unreadable] generated by the Fenton reaction. PCC at a concentration of 0.5 [unreadable]g/ml induced a 50% reduction against the 1,1-diphenyl-2-picrylhydrazyl radical. PCC also induced phase II enzymes as measured by the quinone reductase assay using murine hepatoma cells. The concentration of PCC that induced a doubling of the activity of NAD(P)H: quinone-acceptor oxidoreductase was 15 [unreadable]g/150 [unreadable]l PCC (5 [unreadable]g/ml) was effective in repressing the DNA damaging capacity of 500 pM 2-acetoxyacetylaminofluorene as measured in the Comet assay with CHL cells. These results indicate that commercial by-products from agronomic crops may be a source of novel chemoprotectants.